Blender's version of Sketchup's line tool?
Blender isn't really the right tool for doing precision 3D design, but it can be done.

You will have to do some arithmetic, but one approach would be


start with a view perpendicular to the face you want the edge on.  In your example, it looks like that would be top orthographic, so hit NUMPAD-7
select the knife tool (Shortcut KKEY)
place the cursor on the either of the edges you want to connect and click LMB
type the C key
place the cursor on the other edge and click LMB
type ENTER


This will create the edge you want, but not place it where you want it.


type g
do not move your mouse
Go to transform panel of the properties panel and enter a value.  (if the properties panel is not visible, type N to unhide it) the transform panel looks like




To figure out the value, here is an example, based on your image.
* You appear to be adding the edge to a top face.  This means that you want to set the median y value entry.
* take the y value of point q and subtract 20cm from it.
* enter the result.

You end up with a precisely place edge.